how would you even measure that without the causal effects of "we only hired people from these schools" not being an interfering factor?
my own personal experience (from > 20 years in professional workforce) is that there's generally very little correlation between work performance and academic pedigree. For entry level positions, I've seen a bit, but after, say, 5-8 years, people are generally either really good at what they're doing, and are getting better, or they're struggling. I've not seen any correlation between the strugglers/non-strugglers and where they went to school, or indeed, to some extent, what they studied.